Post on 30-Dec-2015
The Nervous System
Evolution of the Nervous System
Nervous System Evolution
• Overtime, animals developed nervous systems that demonstrate bilateral symmetry, cephalization, and an increasing number of neurons.
Nervous System Evolution• Highly evolved organisms have brains with
three partsHINDBRAIN
operates organs under little conscious control;
coordinates muscle movement
MIDBRAIN
Reflex coordination
FOREBRAIN
Higher – level thinking skills
Nervous system cells
dendrites
cell body
axon
synaptic terminal
• Neuron– a nerve cell
• Structure fits function– many entry
points for signal– one path out– transmits signalsignal direction
signaldirection
dendrite cell body axon synapse
myelin sheath
Transmission of a signal• Think dominoes! – start the signal • knock down line of dominoes by tipping 1st one
trigger the signal– propagate the signal• do dominoes move down the line?
no, just a wave through them! – re-set the system• before you can do it again,
have to set up dominoes again reset the axon
Transmission of a nerve signal• Neuron has similar system– protein channels are set up – once first one is opened, the rest open in
succession• all or nothing response
– a “wave” action travels along neuron – have to re-set channels so neuron can react
again
Cells: surrounded by charged ions• Cells live in a sea of charged ions– anions (negative)• more concentrated within the cell• Cl-, charged amino acids (aa-)
– cations (positive)• more concentrated in the extracellular fluid• Na+
Na+ Na+ Na+ Na+ Na+ Na+ Na+Na+ Na+ K+ Na+ Na+
Cl-
K+ Cl- Cl- Cl-K+
aa-K+ Cl- Cl-
aa- aa-aa-
aa- aa-K+
K+channel leaks K+
channel leaks K+
+–
Cells have voltage!• Opposite charges on opposite sides of cell
membrane– membrane is polarized• negative inside; positive outside• charge gradient• stored energy (like a battery)
+ + + + + + + ++ + + + + + +
+ + + + + + + ++ + + + + + +
– – – – – – – ––– – – – –
– – – – – – – ––– – – – –
How does a nerve impulse travel?• Stimulus: nerve is stimulated– reaches threshold potential
• open Na+ channels in cell membrane• Na+ ions diffuse into cell
– charges reverse at that point on neuron• positive inside; negative outside • cell becomes depolarized
– + + + + + + ++ + + + + + +
– + + + + + + ++ + + + + + +
+ – – – – – – –– – – – – – –
+ – – – – – – –– – – – – – –Na+
Gate
+ –
+
+
channel closed
channel open
How does a nerve impulse travel?• Wave: nerve impulse travels down neuron– change in charge opens
next Na+ gates down the line • “voltage-gated” channels
– Na+ ions continue to diffuse into cell– “wave” moves down neuron = action potential
– – + + + + + +– + + + + + +
– – + + + + + +– + + + + + +
+ + – – – – – –+ – – – – – –
+ + – – – – – –+ – – – – – –Na+
wave
How does a nerve impulse travel?• Re-set: 2nd wave travels down neuron– K+ channels open
• K+ channels open up more slowly than Na+ channels– K+ ions diffuse out of cell– charges reverse back at that point
• negative inside; positive outside
+ – – + + + + +– – + + + + +
+ – – + + + + +– – + + + + +
– + + – – – – –+ + – – – – –
– + + – – – – –+ + – – – – –Na+
K+
wave
How does a nerve impulse travel?• Combined waves travel down neuron– wave of opening ion channels moves down neuron– signal moves in one direction • flow of K+ out of cell stops activation of Na+ channels in
wrong direction
+ + – – + + + ++ – – + + + +
+ + – – + + + ++ – – + + + +
– – + + – – – –– + + – – – –
– – + + – – – –– + + – – – –Na+
wave
K+
How does the nerve re-set itself?• After firing a neuron has to re-set itself– Na+ needs to move back out– K+ needs to move back in– both are moving against concentration gradients• need a pump!!
+ + + + + – – ++ + + + + – –
+ + + + + – – ++ + + + + – –
– – – – – + + –– – – – – + +
– – – – – + + –– – – – – + +Na+
Na+Na+
Na+ Na+Na+
K+K+K+K+ Na+ Na+
Na+Na+Na+
Na+Na+
Na+Na+
Na+
Na+
K+K+K+K+
K+K+
K+ K+
wave
K+
Na+
How does the nerve re-set itself?• Sodium-Potassium pump– active transport protein in membrane• requires ATP
– 3 Na+ pumped out– 2 K+ pumped in– re-sets charge
across membrane
ATP
Neuron is ready to fire againNa+ Na+ Na+ Na+ Na+ Na+ Na+
Na+ Na+ Na+ Na+ Na+ Na+
Na+ Na+ Na+ Na+ Na+ Na+ Na+
Na+ Na+ Na+ Na+ Na+ Na+
K+
K+ K+ K+ K+
K+
aa-K+ K+ K+
aa- aa-aa-
aa- aa-
+ + + + + + + ++ + + + + + +
+ + + + + + + ++ + + + + + +
– – – – – – – –– – – – – – –
– – – – – – – –– – – – – – –
resting potential
1. Resting potential2. Stimulus reaches threshold
potential3. Depolarization
Na+ channels open; K+ channels closed
4. Na+ channels close; K+ channels open
5. Repolarizationreset charge gradient
6. UndershootK+ channels close slowly
Action potential graph
–70 mV
–60 mV
–80 mV
–50 mV
–40 mV
–30 mV
–20 mV
–10 mV
0 mV
10 mV DepolarizationNa+ flows in
20 mV
30 mV
40 mV
RepolarizationK+ flows out
ThresholdHyperpolarization(undershoot)
Resting potential Resting1
2
3
4
5
6
Mem
bra
ne
po
ten
tial
Myelin sheath
signaldirection
Axon coated with Schwann cells• insulates axon• speeds signal
signal hops from node to node saltatory conduction
myelin sheath
myelin
axon
Na+
Na+
++ + + + –
–
action potential
saltatoryconduction
Multiple Sclerosis immune system (T cells)
attack myelin sheath loss of signal
Multiple Sclerosis immune system (T cells)
attack myelin sheath loss of signal
Synapse
Impulse has to jump the synapse!– junction between neurons– has to jump quickly from one cell to
next
What happens at the end of the axon?
How does the wave
jump the gap?
If I roll a:
• 1 – On your own, no notes• 2 – On your own, with notes• 3 – With partner, no notes• 4 - with partner, with notes• 5 – as a class, no notes• 6 – as a class, with notes
Explain the high glucose demands of a neuron.
axon terminal
synaptic vesicles
muscle cell (fiber)
neurotransmitteracetylcholine (ACh)receptor protein
Ca++
synapse
action potential
Chemical synapse Events at synapse
action potential depolarizes membrane
opens Ca++ channels neurotransmitter vesicles fuse
with membrane release neurotransmitter to
synapse diffusion neurotransmitter binds with
protein receptor ion-gated channels open
neurotransmitter degraded or reabsorbed
We switched…from an electrical signalto a chemical signal
Nerve impulse in next neuron • Post-synaptic neuron– triggers nerve impulse in next nerve cell• chemical signal opens ion-gated channels • Na+ diffuses into cell• K+ diffuses out of cell
– switch back to voltage-gated channel
– + + + + + + ++ + + + + + +
– + + + + + + ++ + + + + + +
+ – – – – – – –– – – – – – –
+ – – – – – – –– – – – – – –Na+
K+
K+K+
Na+ Na+
Na+
ion channel
binding site ACh
Neurotransmitters• Acetylcholine – transmit signal to skeletal muscle
• Epinephrine (adrenaline) & norepinephrine– fight-or-flight response
• Dopamine– widespread in brain– affects sleep, mood, attention & learning– lack of dopamine in brain associated with Parkinson’s
disease– excessive dopamine linked to schizophrenia
• Serotonin– widespread in brain– affects sleep, mood, attention & learning
snake toxin blockingacetylcholinesterase active site
Acetylcholinesterase
acetylcholinesterase
active site in red
neurotoxin in green
• Enzyme which breaks downacetylcholine neurotransmitter – acetylcholinesterase inhibitors = neurotoxins
• snake venom, sarin, insecticides
Questions to ponder…
• Why are axons so long?• Why have synapses at all?• How do “mind altering drugs” work?– caffeine, alcohol, nicotine, marijuana…
• Do plants have a nervous system?– Do they need one?